image.bbclass: Add MLPREFIX to DEPENDS

Add MLPREFIX to DEPENDS to ensure the correct qemuwrapper is dependended upon. Its searched
for in PATH so we need to do this to ensure the correct version is present.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
qemu.bbclass: Use the correct qemu binary in multilib cases

For example with a lib32 multilib, we need to still use the 64 bit
qemu binary in case we do encounter a 64 bit binary.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>